1. Field of the Invention
The invention generally relates to computer database systems. More particularly, the invention relates to techniques for optimization of database queries containing user-defined functions.
2. Description of the Related Art
Databases are well known systems for storing, searching, and retrieving information stored in a computer. One type of database used today is the relational database, which stores data using a set of tables that may be reorganized and accessed in a number of different ways. Users access information in relational databases using a relational database management system (DBMS).
Each table in a relational database includes a set of one or more columns. Each column typically specifies a name and a data type (e.g., integer, float, string, etc.), and may be used to store a common element of data. For example, in a table storing data about patients treated at a hospital, each patient might be referenced using a patient identification number stored in a “patient ID” column. Reading across the rows of such a table would provide data about a particular patient. Tables that share at least one attribute in common are said to be “related.” Further, tables without a common attribute may be related through other tables that do share common attributes. A path between two tables is often referred to as a “join,” and columns from tables related through a join may be combined to form a new table returned as a set of query results.
A query of a relational database may specify which columns to retrieve data from, how to join the columns together, and conditions (predicates) that must be satisfied for a particular data item to be included in a query result table. Current relational databases require that queries be composed in query languages. A widely used query language is Structured Query Language (SQL). However, other query languages are also used.
Once composed, a query is executed by the DBMS. Typically, the DBMS interprets the query to determine a set of steps (hereafter referred to as a “query plan”) that must be carried out to execute the query. However, in most cases, there are alternative query plans that can be carried out to execute a given query. Thus, the DBMS often includes a query optimizer, which selects the query plan that is likely to be the most efficient (i.e., requiring the fewest system resources, such as processor time and memory allocation).